Many of parts for automobiles are manufactured by press forming a steel sheet, which is one of metal sheets. In recent years, since thinner steel sheets are used in order to reduce weights of parts for automobiles, there is a need for their higher strength. However, the higher strength creates a problem that deformation from a desired shape is increased due to springback after press forming, and thus countermeasures against the springback become necessary.
“Metal sheet” herein means a hot rolled steel sheet, a cold rolled steel sheet, or a surface treated steel sheet resulting from a surface treatment (an electro-galvanizing process, a hot-dip galvanizing process, an organic coating process, or the like) on the steel sheet. Further, a metal sheet may be a sheet made of any of various metals, such as a ferritic stainless steel, an austenitic stainless steel, an aluminum alloy, or a magnesium alloy.
At present, for a countermeasure against springback, numerical simulation by a finite element method (hereinafter, FEM) is used the most. An example of a countermeasure against springback using a numerical simulation by an FEM is as follows. First, a springback analysis by an FEM is performed, and based on a result of the springback analysis, a factorial analysis of the springback is performed. Next, based on a result of the springback factorial analysis, a countermeasure is implemented, and effects of the implementation of the countermeasure are checked by the FEM again. This procedure is repeated until a desired shape is obtained, and thereafter, an actual die of press forming is manufactured.
As a factorial analysis of springback by an FEM, there is, for example, a method disclosed in Patent Literature 1. The factorial analysis of springback of Patent Literature 1 clarifies an influence exerted on springback by a residual stress acting on a press forming product (before die release) after a press forming analysis. Specifically, by this technique, a springback analysis result, which is obtained by performing a springback analysis by partially changing a residual stress distribution after the press forming analysis, is compared with a springback analysis result obtained by performing a springback analysis without changing the residual stress distribution. Thereby, an influence of the changed residual stress distribution is checked. If, as a result of doing this, an influence of a residual stress of a particular portion is able to be clarified, and the influence is identified as affecting the springback, countermeasures against the springback are able to be planned.
As a countermeasure against springback, for example, there is a method of giving, by adding a new shape to a press forming product, a tensile stress to that portion that has been added with the shape. Or, for press forming products manufactured by performing two steps of press working, there is a method or the like of giving a compressive stress thereto by adding an embossed or bead shape in the first step and thereafter crushing and stretching that shape in the second step.
For implementation of such a countermeasure against springback, how a modification is performed to which part of a press forming product is important. In order to clarify the part of the press forming product where the modification is performed and the method of the modification, it is important to evaluate the amount of springback correctly. In general, an amount of springback is evaluated after specifying a portion of a press forming product and determining an index, such as an evaluation direction or the like.
Deformation due to springback is roughly classified into camber, torsion, and sectional opening (closing). Of these, as to the sectional opening, the deformation itself is easy to be understood, and is comparatively easy to be dealt with by putting a forecast in the die beforehand. However, since camber and torsion occur in a combined manner in actual springback, it is more difficult to determine an index for evaluating the amount of the springback. In particular, if a part is large, dealing with them by putting a forecast in the die beforehand is often difficult. Therefore, it is important to clearly evaluate each of the camber and torsion quantitatively. However, torsion is particularly difficult to be evaluated quantitatively, and thus clarifying an index for the evaluation is also difficult.
Accordingly, as a conventional technique focused on torsion, for example, a method of calculating a center of gravity in an evaluated section and calculating a torsional torque of the section corresponding to rotation of the section about that center of gravity is disclosed in Patent Literature 2. By this technique, torsion is solved by eliminating the torsional torque causing the torsion.